The invention relates generally to dye transfer printers such as thermal printers, and in particular to the problem of crease or wrinkle formation in successive dye transfer areas of a dye donor web. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color dye donor web that is used in a thermal printer is substantially thin and has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent colorless laminating section immediately after the cyan color section.
Each color section of the dye donor web consists of a dye transfer area that is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are not used for printing. The dye transfer area is about 95% of the web width and the two edge areas are each about 2.5% of the web width.
To make a multi-color image print using a thermal printer, a motorized donor take-up spool pulls the dye donor web from a donor supply spool in order to successively draw an unused single series of yellow, magenta and cyan color sections over a stationary bead of selectively heated resistive elements on a thermal print head between the two spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred, via the bead of selectively heated resistive elements, in superimposed relation onto a dye receiver such as a paper or transparency sheet or roll, to form the color image print. The bead of resistive elements extends across the entire width of a color section, i.e. across its dye transfer area and the two edge areas alongside the transfer area. However, only those resistive elements that contact the dye transfer area are selectively heated. Those resistive elements that contact the two edge areas are not heated. In other words, the dye transfer is effected from the dye transfer area to the receiver medium, but not from the two edge areas to the receiver medium.
As each color section, including its dye transfer area and the two edge areas alongside the transfer area, is drawn over the bead of selectively heated resistive elements, the color section is subjected to a longitudinal tension particularly by a pulling force of the motorized donor take-up spool. Since the dye transfer area is heated by the resistive elements, but the two edge areas alongside the transfer area are not, the transfer area is significantly weakened and vulnerable to stretching as compared to the two edge areas. Consequently, the longitudinal tension will stretch the dye transfer area relative to the two edge areas. This stretching causes the dye transfer area to become thinner than the non-stretched edge areas, which in turn causes creases or wrinkles to develop in the transfer area, particularly diagonally across those regions of the transfer area that are close to the two edge areas. The creases or wrinkles are most notable in the regions of the dye transfer area that are close to the two edge areas because of the sharp, i.e. abrupt, transition between the weakened transfer area and the stronger edge areas, and they tend to be slanted diagonally across such regions.
As the dye donor web is pulled by the motorized donor take-up spool over the bead of selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing or rear end portion of a used dye transfer area at least to a leading or front end portion of the next dye transfer area to be used. A problem that can result is that creases or wrinkles in the leading or front end portion of the next dye transfer area to be used will cause undesirable line artifacts to be printed on a leading or front end portion of the dye receiver, when the dye transfer occurs at the creases in the leading end portion of the next dye transfer area to be used. The line artifacts printed on the dye receiver are relatively short, but quite visible.
The question presented therefore is how to solve the problem of the creases or wrinkles being created in an unused transfer area so that no line artifacts are printed on the dye receiver during the dye transfer.
According to one aspect of the invention, there is provided a thermal printer capable of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver. The printer comprises:
a thermal print head for heating the dye transfer area of the dye donor web sufficiently to effect a dye transfer from the dye transfer area to the dye receiver, but not heating two opposite edge areas of the dye donor web alongside the dye transfer area sufficiently to effect a dye transfer from the two edge areas to the dye receiver, so that the dye transfer area is vulnerable to being stretched relative to the two edge areas;
a web take-up that takes up the dye donor web, and which exerts a pulling force on the dye transfer area and two edge areas at the print head that is sufficient to stretch the dye transfer area relative to the two edge areas to possibly form slanted creases in the dye transfer area diagonally extending at least across respective regions close to the two edge areas; and
a web spreader adapted to spread the dye donor web diagonally across at least the regions in which slanted creases can form in order to oppose crease formation that can occur when the pulling force is exerted, whereby crease formation can at least be substantially prevented.
According to another aspect of the invention, there is provided a method of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver in a dye transfer printer. The method corresponds to operation of the thermal printer as described above.